Context : While the search for exoplanets around main sequence stars more massive than the Sun have found relatively few such objects , surveys performed around giant stars have led to the discovery of more than 30 new exoplanets .
The interest in studying planet hosting giant stars resides in the possibility of investigating planet formation around stars more massive than the Sun .
Masses of isolated giant stars up to now were only estimated from evolutionary tracks , which led to different results depending on the physics considered .
To calibrate the theory , it is therefore important to measure a large number of giant star diameters and masses as much as possible independent of physical models .

Aims : We aim in the determination of diameters and effective temperatures of 5 giant stars , one of which is known to host a planet .
We used optical long baseline interferometry with the aim of testing and constraining the theoretical models of giant stars .
Future time-series spectroscopic observations of the same stars will allow the determination of masses by combining the asterosimological analysis and the interferometric diameter .

Methods : AMBER/VLTI observations with the ATs were executed in low resolution mode on 5 giant stars .
In order to measure high accurate calibrated squared visibilities , a calibrator-star-calibrator observational sequence was performed .

Results : We measured the uniform disk and limb-darkened angular diameters of 4 giant stars .
The effective temperatures were also derived by combining the bolometric luminosities and the interferometric diameters .
Lower effective temperatures were found when compared to spectroscopic measurements .
The giant star HD12438 was found to have an unknown companion star at an angular separation of \sim 12 mas .
Radial velocity measurements present in the literature confirm the presence of a companion with a very long orbital period ( P \sim 11.4 years ) .

Conclusions :